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| Titel |
Constraining the timing of orogenesis in Shetland using garnet and mica
geochronology |
| VerfasserIn |
Stephanie Walker, Matthew Thirlwall, Rob Strachan, Anna Bird |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250137638
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| Publikation (Nr.) |
 EGU/EGU2017-417.pdf |
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| Zusammenfassung |
| The archipelago of Shetland is uniquely placed within the Caledonides of the North Atlantic
due to its central location between the East Greenland, Norwegian, and Scottish sectors of the
orogen1.
Here we present Lu-Hf and Sm-Nd garnet ages, and Rb-Sr mica ages for metamorphic
lithologies from a wide geographic and tectonostratigraphic area of Shetland. Lu-Hf and
Sm-Nd ages are broadly interpreted to represent the timing of peak metamorphic
conditions in garnets where prograde zoning is observed in trace-element LA-ICPMS
profiles. Conversely, Rb-Sr mica ages are interpreted to relate to deformation-induced
recrystallization, and typically yield similar ages to Sm-Nd garnet ages.
Ordovician-Silurian garnets are prevalent throughout Shetland and span the entire
collisional orogenic: from the initiation of tectonism by obduction of the Shetland
ophiolite at 489±7 Ma, to the oblique collision of Laurentia and Baltica at c. 427
Ma. The majority of garnets grew during the Grampian phases of the Orogen. In
the northern Isles of Unst and Fetlar, peak metamorphism appears to have been
reached up to 10 Ma earlier than in mainland Scotland, with several Lu-Hf and
Sm-Nd ages of c. 480 Ma, between only 9-4 Ma after the obduction of the Shetland
ophiolite2. Throughout all of Shetland, ages of c.450 Ma provide further evidence
for the Grampian II accretionary event recorded in the Moine nappe of mainland
Scotland3.
Only two samples yield garnet ages that relate to the Silurian Scandian phase of
orogenesis that is widespread in both the East Greenland and Norwegian Caledonides. This
may indicate that Shetland had been uplifted to higher in the nappe pile than these areas
during the later phases of the orogenic cycle. However, Devonian Rb-Sr mica ages in
westernmost Shetland are consistent with the diachronous northward-younging of the thrust
belts that define the western Laurentian margin, suggesting a late orogenic evolution that has
more in common with East Greenland and Norway than with northern Scotland4, where
movement on the foreland propagating nappe systems had essentially ceased by 429
Ma5.
1Fossen 2010. GSLSP 335
2Crowley & Strachan 2015. JGSL 172
3Bird et al 2013. JGSL 170
4Walker et al 2016. JGSL 173
5Goodenough et al 2011. JGSL 168 |
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